Proble with CPU / Motherboard

I have a very strange problem with my computer. Here is some technical information about it:
CPU: AMD Athlon 650 MHz, Slot A
Mortherboard: Asus K7M
Memory: 128 Mb, PC100

A few weeks ago I run a test on it using Sandra program. The test showed that my processor can handle only 205 MIPS and 232 MFLOPS, which is almost 6 times slower than ADM Duron 600 MHz can handle! My friend run the same test on his computer, he has Intel Celeron 468 MHz processor, and it was also (about 4 times) faster, than mine!
The next day I went back to the shop and got an another processor (Athlon 600 MHz) jus for a test. I replaced my processor with the new one and run the same test again. I was surprised, when it showed almost the same results (a bit slower (50 MHz)) as with old processor. So the problem is not in the processor, but somewhere else!
I checked all the settings on the motherboard, but didn't find any errors.

Could someone help me with this? Should I take the motherboard back to the store?

.thanks.

Timo Radtsenko (sco.bl@sci.fi)